1. Field of the Invention
This disclosure relates to tracking the position and orientation of an object moving within a three-dimensional spatial region, using a set of point source emitters that output respective optical emissions within the field-of-view (FOV) of a sensor array comprising one or more optical angle of arrival sensors.
2. Description of the Related Art
Tracking the movement and/or orientation of an object quickly and precisely is critical for a variety of military and civilian applications. The efficacy of tracking systems is often based on the detection and processing of several parameters related to movement and orientation. These parameters vary depending on the implementation, and can include magnetic, acoustic and inertial measurements that reveal the position and pose or orientation of the tracked object. Current solutions are limited due to their susceptibility to noise or imprecise measuring and processing techniques of these parameters. Conventional tracking systems that employ magnetic sensors are strongly affected by surrounding metallic structures and moving metal in the measurement area. Currently implemented optical sensing systems similarly suffer from undesirable sunlight effects and often have low resolution that is limited by the need to view illuminated patterns on the object being tracked. Inertial measurement units (IMUs) experience undesirable IMU drift, and acoustic sensing systems are limited by their low acoustic update rate.
A need exists for accurate and low latency methods to determine and track the position and orientation (pose) of an object within a defined environment. For example, in an aircraft cockpit there are many applications where the pilot's helmet includes a head mounted display that provides situational awareness where, by tracking the pilot's helmet orientation, the system directs the view of external sensors. Alternatively, the orientation of the pilot's helmet may be used to direct (cue) the aim point of a weapon system. A helmet tracking system may also be used in various ground and underwater vehicles to provide the driver or occupant with a sensor-based view of the external environment with a head-directed viewing direction. Other applications for an object tracking system include assembly operations as in architectural construction or assembly of large systems such as aircraft. There are also applications for use with remote medical (surgical) equipment.
Various methods have been developed for object tracking systems. For example, helmet tracking methods include the use of magnetic sensors (see, e.g., “Adaptive magnetic tracker—A revolution in electro-magnetic tracker technology,” Y. Kranz, G. Kornblau, S. Stoker, SPIE Proceedings, Vol. 5442, pp. 149-156 (2004)), ultrasonic (acoustic) sensors (see, e.g., U.S. Pat. No. 6,141,293), miniature inertial sensors, and optical sensors or combinations of these. Magnetic sensors have the features of accuracy, speed, detection range, and small size. They, however have the significant disadvantage in that their accuracy requires precision mapping of the magnetic environment in the cockpit where most metal objects will distort the magnetic field distribution. This mapping is time consuming and troublesome and can be susceptible to changes in the environment (moving metal or relocated equipment). Ultrasonic methods have rather low update rates because multipath (echo) effects require an extended time between measurements. Miniature inertial sensors, for example Microelectromechanical Systems (MEMS) devices, suffer from relatively short term drift and require frequent recalibration from another type of sensor in a hybrid architecture.
Markos (“All-optical helmet tracker for multi-craft multi-vehicle systems”, C. T. Markos, J. J. Atkinson, G. Wyntjes, SPIE proceedings, Vol. 5079, pp. 86-94 (2003)) describes one scheme that includes several optical transmitters positioned within the cockpit together with receivers mounted on the helmet. An optical phase measurement method is used to determine the distance from each helmet receiver from the transmitter source. Processing this information, this approach achieves angular resolution of 3 mrad and position resolution to 250 microns. However, the data update rate of 125 Hz can introduce sufficient latency into the system to cause errors in targeting. Also the normal expansion and contraction of the crew station (cockpit) causes the locations of the transmitters (emitters) to change. This may result in significant measurement error.
The optical tracking method of Tawada and Hirooka (“A new optical HMT system based on image processing,” K. Tawada and K. Hirooka, SPIE Proceedings, Vol. 6955, 69550A-1 to 69550A-11 (2008)) is based on the combination of image processing based optical sensing, integrated with inertial sensors. The optical image processing method includes multiple sensors that detect markers that are located in the environment. The image of these markers is processed to determine the relative range, position, and orientation between the marker and the sensor. This type of approach requires the processing of 2D images and is claimed to result in position error to better than one pixel. However, processing of a complete 2D image can be time consuming, which can be an issue for applications requiring rapid response. Additionally, the field of view of the optical sensors is limited in this situation to a distance necessary for the sensors optics to capture the entire image. This severely limits the size of the three dimensional area being measured.
Odell and Kogan (“Next generation, high accuracy optical tracker for target acquisition and cueing,” D. S. Odell and V. Kogan, SPIE Proceedings, Vol. 6224, 62240C-1 to 62240C-10 (2006)) describe yet another optically based tracker. Their method measures the angle between an array of point source emitters (LEDs) mounted on a helmet and multiple optical sensors that are placed in the cockpit. Each sensor comprises a linear detector array and employs “a transmissivity mask which is located a known distance above the linear detector.” The mask is a superposition of three periodic patterns which is imaged onto the linear detector. Analysis of the pattern on the detector arrays determines the angle of each emitter in a plane. Multiple measurements using sensors having their linear detector arrays oriented in orthogonal directions result in a determination of the position and orientation of the helmet. This system suffers from extraneous sources of light that are frequently present in the cockpit in which it is operated.